1. Technical Field
The present disclosure relates to an optical receiver having an apparatus for varying decision threshold level and an optical transmission system having the same. More specifically, the present disclosure relates to an optical receiver having an apparatus for varying decision threshold level capable of enhancing a transmission characteristic by adjusting a decision threshold level depending on power inputted into the optical receiver, using that a noise characteristic of level 1 may vary depending on a difference in wavelengths between a filtered incoherent light source and Fabry-Perot Laser Diode (F-P LD), and an optical transmission system having the same.
2. Discussion of Related Technology
Demand on capacity extension at an existing optical network has been required for meeting the gradually increasing data traffics and for providing a variety of multimedia services. In the existing copper-based optical network, there is a limit in bandwidth available for each subscriber. As a method to solve the above problem, a study on passive optical network (PON) technologies based on optical fiber has actively carried out, where PON is flexible and transparent, and is possible to use a broad bandwidth.
Currently, a wavelength division multiplexing (WDM) among PON technologies has been perceived to be an ultimate solution. In a WDM, one wavelength must be assigned to each subscriber and thus the essential factor in a wavelength division multiplexing-passive optical network (WDM-PON) technology is to embody a system for WDM-PON by using a low-cost transceiver. A wavelength locked F-P LD which has been suggested recently is recognized as an economic light source for a WDM-PON. Here, the wavelength locked F-P LD is a light source obtained by injecting a filtered incoherent light source or a broadband light source (hereinafter referred to BLS) into a F-P LD which is oscillated in a multi-mode and locking a wavelength of the F-P LD to be a wavelength of the filtered BLS so that the F-P LD is oscillated in a quasi-single mode.
However, the wavelength of F-P LD may vary as ambient temperature varies and the varied wavelength of F-P LD does not conform to the wavelength of a filtered incoherent light source, which leads to a degradation in a transmission quality of optical signals.
FIG. 1 illustrates a structural view of an optical receiver being used in an exemplary optical transmission system. As illustrated in FIG. 1, an optical receiver being used in an exemplary optical transmission system is comprised of a photo diode (PD) 441 for receiving an optical signal and for transforming the received optical signal into an electrical signal, a trans-impedance amplifier (TIA) 442 for pre-amplifying the electrical signal transformed by PD 441, a limiting amplifier (LA) 443 for deciding the electrical signal amplified by TIA 442 as either level “0” or level “1” and for amplifying and outputting the decision signal, and a clock and data recovery (CDR) 444 for generating a clock and data from the amplified decision signal.
LA 443 is an amplifier having a high gain and performs a function that compares an input signal with a reference value corresponding to a decision threshold level and decides the input signal as level “0” (low level) or level “1” (high level). That is, an input signal is represented as level “0” if the input signal is lower than a reference value, while being represented as level “1” if the input signal is higher than a reference value. When deciding by comparing a reversed optical signal with a reference value depending on an optical transmission system, LA 443 decides an input signal as level “0” if the input signal is lower than a reference value and decides an input signal as level “1” if the input signal is higher than a reference value. Generally, in an optical transmission system, noise of level “1” is greater than noise of level “0” due to using an optical amplifier and thus a desirable transmission characteristic of an optical signal can be obtained when a decision threshold level (i.e., a reference value) must have a lower voltage value as optical signal power has a higher value.
Further, the transmission characteristic of an optical signal described above can be applied likewise to an optical transmission system which uses, as its light source, incoherent light such as BLS or a light emitting diode (LED). In case of a wavelength-locked F-P LD which will be described as an embodiment of the present invention, a wavelength-locked F-P LD is the most complicated case because the wavelength of a wavelength-locked F-P LD may vary depending on the temperature thereof so that optimal decision threshold level may vary depending on a difference in wavelength between a filtered incoherent light source and an F-P LD and therefore a characteristic depending on temperature must be also considered.
The discussion in this section is to provide general background information, and does not constitute an admission of prior art.